1. Field of the Invention
The present invention relates to an optical equalizer available for a very-high-speed long-distance transmission line in optical communications.
2. Description of the Related Art
As is well known, optical communications need technical advances so as to fulfill a major role in communication infrastructures in intellectual information (informationized) society. For this reason, the optical communications technology has made rapid advance, and its transmission rate and degree of multiplexing are improving with every passing year. Thus, trunk communication lines built by the use of high technology will soon become insufficient in capacity, requiring improvements by the state-of-the-art technology.
For improvements of the trunk communication lines, alterations only to terminal stations would be economically advantageous. However, rebuilding of optical fiber lines used as trunk lines because of their oldfashioned responses is very costly, which is almost impossible. It is therefore desired to develop methods which can realize much higher speed transmission with existing lines.
As a concrete example, consider now that such improvements as to enable much higher speed transmission are made in long-distance trunk lines, which are employed at a transmission rate in the order of giga bits per second (Gbps), using optical fibers having wavelength characteristics in which the delay time dispersion is minimum in the 1.3-.mu.m band. (Hereinafter, such optical fibers are referred to as normal dispersive fibers.) In this case, in order to keep light transmission loss small and secure light received power required, the wavelength of light used may be changed to 1.55 .mu.m. In addition, intermediate light repeaters for light amplification may be provided.
However, the normal dispersive fibers have a delay-time dispersion as large as 17 ps/nm in the 1.55-.mu.m band, which will cause a deficiency of bandwidth. Thus, the use of direct modulation of semiconductor lasers that is currently considered effective would make difficult very high-speed long-distance transmission at a required rate of several gigabits per second. In this case, the use of external light modulators might be considered to reduce a modulation bandwidth required and alleviate the influence of the delay time dispersion. However, this method will not only increase cost but also confront a band limitation problem, failing to meet the requirements of the very-high-speed long-distance transmission.
On the other hand, dispersion-shifted optical fibers in which the delay time dispersion is minimum in the 1.55 .mu.m band can be used as trunk lines. These optical fibers, used as trunk lines, are connected by light amplifiers, forming a transmission line. Through this transmission line, light signals can be transmitted without being converted into electric signals and can, hence, be transmitted at a very high speed and over a very long distance (for example, over the Pacific). Since the band used in this high-speed, long-distance transmission line is in the neighborhood of the zero point of the parabolic delay-time dispersion characteristic of the fibers, a bandwidth deficiency problem arises. Solution of this problem would need complicated control, resulting in an increase in transmission cost.